1. Field of the Invention
The present invention relates to a method of removing an insulating film or a protective film formed on wirings in a producing method of forming semiconductor devices such as thin film transistors (TFTs) and thin film diodes (TFDs) on an insulating substrate such as a glass substrate or other various substrates, or of forming a thin film integrated circuit, particularly an active matrix liquid crystal display device, using such semiconductor devices.
2. Description of the Related Art
Liquid crystal display devices are used in many TV sets, word processors, etc. as thin and light weight display devices. In particular, active matrix liquid crystal display devices in which a thin film transistor (TFT) and other elements are formed at each display pixel by utilizing the producing technology of integrated circuits such as ICs and LSIs are expected as liquid crystal display devices capable of displaying superior images.
As the area of active matrix display devices using TFTs is increased, they are required to have small wiring resistance. The resistance of chromium (Cr) which has conventionally been used for a wiring is high. In general, when a low-temperature-processing polycrystalline silicon thin film, which is superior in high speed operation to an amorphous silicon thin film, is used as a semiconductor layer, since a producing process includes a doping process and a laser irradiation process, it is an important point whether gate electrode-wirings are sufficiently durable in such process.
Anodized aluminum is promising as a material that satisfies such conditions as low resistance and superior process durability.
FIGS. 1A-1D show a producing process for forming an electrode or wiring 11 containing mainly aluminum on a desired base member (object) such as an insulator, an insulating film, or a semiconductor.
In FIG. 1A, an electrode or wiring 11 containing mainly aluminum is formed on a base member 10 by sputtering, evaporation or the like. An oxidized aluminum layer (oxide layer) 12 is then formed on the aluminum surface by anodization in an electrolytic solution using the electrode or wiring 11 as an anode. The oxide layer 12 is used to improve a withstanding voltage or form offset gate regions of a TFT. In FIG. 1B, after the formation of the oxide layer 12, a silicon oxide or silicon nitride film is formed as an interlayer insulating film 13.
Since it is necessary to form a wiring for contact to the electrode or wiring 11, a hole portion 15 is formed by forming a desired mask 14 (usually a resist is used) and then etching a part of the electrode or wiring 11 with an etchant (etching material) such as buffer hydrofluoric acid (BHF), as shown in FIG. 1C. After the formation of the hole 15, the mask 14 is removed and a desired metal wiring 16 as a contact to the electrode or wiring 11 is deposited by evaporation or sputtering. (FIG. 1D).
Usually, an active matrix liquid crystal display device has a configuration in which a liquid crystal material is held between two glass substrates and TFTs are formed on one of the glass substrates. One of the conditions required for the substrate on which the TFTs are to be formed is a low degree of alkali metal dissolving. This is because if a substrate, such as a soda lime glass, containing a large amount of Na, K, etc. is used, alkali metals such as Na and K are dissolved during formation of a TFT substrate, so that the TFT characteristics are much deteriorated. For this reason, low alkali glass is generally used which contains a small amount of alkali metals.
Another condition is that contraction of a substrate which occurs after a heat treatment of the TFT substrate formation process is small. This is because if contraction of a substrate is large after a heat treatment, it becomes difficult to position alignment marks of a patterning mask in a subsequent photolithography process. In the photolithography process, a maximum allowable value of alignment mark deviation due to substrate contraction is 2 .mu.m or less. This value corresponds to changes in 20 ppm and 4 ppm in the case of substrates of 100 mm square and 500 mm square, respectively. In a process of producing a TFT substrate, a heat treatment of 350.degree. C. and 3 hours is needed for an amorphous silicon TFTs and a heat treatment of 600.degree. C. and 4 hours is needed for low-temperature-processing polycrystalline silicon TFTs. Glass substrates satisfying the above conditions for such a heat treatment are needed.
Examples of glass materials that satisfy the conditions of (1) a low alkali content and (2) superior heat resistance and that are now used in active matrix liquid crystal display devices are #7059 and #1737 of Corning Glass Works and NA45 of Nippon Electric Glass Co., Ltd. In particular, #1737 is promising as a substrate having a small contraction amount after a heat treatment because it has a higher strain point than other glass materials. Although aluminum can be anodized easily, it is difficult to remove an aluminum oxide by etching.
Conventionally, etching removal of an aluminum oxide (i.e., alumina) film, is generally performed by using a phosphoric acid solution containing chromic acid anhydride. (See "Testing method of thickness of anodic oxide coatings of aluminum and aluminum alloys" of JIS H8680 and "Anodizing process standard of aluminum and aluminum alloys" of JIS H9500.) However, there remain many problems in terms of productivity, partly because chromic acid is a pollution-causing substance and therefore it requires cumbersome waste liquid processing.
Thus, as an alumina etching method not using chromic acid, there has been studied a method of using BHF in which 50%-hydrofluoric acid on the market and 40%-ammonium fluoride are mixed with each other at a ratio of 1:6 to 1:100 or BHF containing acetic acid (ABHF). BHF is usually used to etch a silicon oxide film etc. and ammonium fluoride is added to prevent resist peeling. However, the use of these etchants causes deposition of regular and rectangular crystals of 1 to 20 .mu.m in size on an alumina surface. As a result, alumina in deposition portions is not etched at the same rate as that in the other portions, thereby leaving protrusions. This makes it difficult to form stable semiconductor circuits partly because of poor conditions of films formed thereon.
The use of BHF or ABHF causes deposition of regular and rectangular crystals of 1 to 20 .mu.m in size on the surface of the above #1737 substrate, which is thereby clouded. However, when etching is performed with conventional BHF, this phenomenon is not found in the above #7059 substrate, which is commonly used as a substrate for low-temperature-processing TFTs.
To consider a cause of the above phenomenon, Table 1 shows compositions of #7059 and #1737.
TABLE 1 ______________________________________ Compositions of low alkali glass substrates (%) #7059 #1737 ______________________________________ SiO.sub.2 47.7 57.3 BaO 9.9 9.9 Al.sub.2 O.sub.3 11.7 16.7 B.sub.2 O.sub.3 13.3 7.6 CaO &lt;0.1 4.2 SrO &lt;0.1 1.8 MgO &lt;0.1 0.7 NaO &lt;0.1 &lt;0.1 Fe.sub.2 O.sub.3 &lt;0.1 &lt;0.1 K.sub.2 O &lt;0.1 &lt;0.1 ______________________________________
As seen from Table 1, #1737 has a larger content of alumina (Al.sub.2 O.sub.3) than #7059. Taking into consideration the fact that the similar phenomenon occurs in etching an alumina film, protrusions are considered to occur at portions containing a large amount of alumina.
No protrusions occur if DHF, which is produced by diluting hydrofluoric acid with pure water by a factor of 10 to 100, is used as an etchant instead of the above BHF. Since a ratio of an etching rate of DHF to alumina/aluminum of gate wiring is 1 to 2, it can sufficiently be used to form a contact hole by time control. However, it has been impossible to use DHF in a TFT substrate producing process because of resist peeling from a substrate during etching.